Asteroid and Lunar Environment Chamber (alec): Simulated Asteroid and Lunar Environments for Measuring Analog Materials

Introduction: Thermal infrared (TIR) emissivity spectral measurements of planetary surfaces have diagnostic features indicative of rock and mineral compositions. These include: (1) the Christiansen feature (CF), an emissivity maximum resulting from a rapid change in the refractive index at wavelengths just short of the fundamental molecular vibration bands, (2) the reststrahlen bands (RB), the fundamental molecular vibration bands due to Si-O stretching and bending motions, and (3) transparency features (TF), emissivity minima caused by volume scattering in a spectral region of relative transparency between the principal RB. Conel [1] found that the CF position in silicates is diagnostic of mineralogy and average composition and changes with the change in bond strength and molecular geometry associated with changing mineralogy. Previous laboratory emissivity measurements of particulate rocks and minerals under vacuum and lunar-like conditions [2,3,4,5,6,7] observed a shift in CF position to shorter wavelengths (higher wavenumbers), an enhancement in the spectral contrast of the CF maximum, and a decrease in the spectral contrast of the RB. The RB absorption positions, shapes and intensities, and number of absorptions are dependent on the masses, geometry, and bond strengths between anions and cations within a crystal lattice. Thus, each mineral has a diagnostic set of RB absorptions owing to each mineral’s unique composition and/or crystal structure [1,3,8]. TF, like the CF, are indicators of mineral compositions and there is a higher-probability of determining specific rock type compositions if both the CF and TF are used for interpretations [3,9]. As particles decrease in size, the spectral contrast of the TF increases. Donaldson Hanna et al. [7] showed that under a simulated lunar environment the TF of some silicate minerals disappeared. These lab studies demonstrate the high sensitivity of TIR emissivity spectra to environmental conditions under which they are measured and provide important constraints for interpreting new thermal infrared datasets of the Moon, including the Diviner Lunar Radiometer Experiment onboard NASA’s Lunar Reconnaissance Orbiter, as well as telescopic observations and future missions like OSIRIS-REx to asteroids. Laboratory emissivity measurements of minerals, rocks, meteorites, and lunar soils made under lunarand asteroid-like conditions at TIR wavelengths are necessary for the analysis of TIR data sets. However, a complete database of laboratory emissivity spectra measured from the visible-to-near infrared (VNIR) to TIR wavelengths of minerals, rocks, meteorites, and lunar soils of varying compositions and particle sizes has not previously existed. The new Asteroid and Lunar Environment Chamber (ALEC) at Brown University will provide the capabilities to start building those necessary spectral libraries. Instrument Design and Experimental Setup: ALEC is the newest addition to Brown University’s Reflectance Experiment Laboratory (RELAB) [10]. The new environment chamber was built to simulate the temperatures and pressures experienced on lunar and asteroid surfaces. ALEC is a vacuum chamber that can reach pressures of <10 torr. An internal rotate stage enables the measurement of six samples and two blackbodies without breaking vacuum. The rotation stage is covered by a radiation shield that cools it conductively. Each sample cup is covered by a cooled radiation shroud that is painted with high emissivity paint, emulating the coldness of space. Sample cups can be heated from below using temperature controllers [Lakeshore Cryotronics Inc., model 340] with a cup temperature accuracy of 0.1K and/or heated from above using a halogen light source that replicates solar style heating. Both sample cups and blackbodies are monitored by temperature controllers allowing accurate temperature control and monitoring.